The present invention relates in general to optical communication and networking systems and, more particularly, to bit synchronization for high speed data transmission in optical communication and networking systems.
With the advent of high speed data communications, optical transmission media are becoming increasingly common. For high bandwidth applications, for example, fiber optic cable and hybrid fiber-coaxial cable are often the transmission media of choice. The well-known advantages of such optical transmission systems include high bandwidth, high transmission capacity, and high noise immunity, resulting in very high data transmission speeds and low error rates.
The use of optical transmission media has also engendered a need for optical switching devices, optical routing devices, and optical logic gates. Typically in the prior art, the data or other information for optical transmission has been interconverted between optical and electronic forms. For example, data for transmission is typically generated and packaged electronically, then converted into laser pulses for optical transmission. Again, at the receiving end, the received light pulses are converted into an electronic form for further processing in, for example, a personal computer, a workstation, a server, or a router.
Various optical devices have been developed, however, which diminish the need for such optical-electronic interconversions. For example, an optical device referred to as a xe2x80x9cTOADxe2x80x9d has been developed which performs the logical AND function. As another example, this can be used to implement an optical demultiplexer, which allows for selection of one or more channels from time division multiplexed channels, with one channel selected per optical AND gate. These optical gates typically utilize a clock pulse periodically ANDed with selected bits of an optical time division multiplexed (xe2x80x9cOTDMxe2x80x9d) channels to produce an output bit stream from the selected channel. See, e.g., Prucnal et al. U.S. Pat. No. 5,493,433, xe2x80x9cTerahertz Optical Asymmetric Demultiplexerxe2x80x9d. For high speed data transmission, for example, an OTDM transmission in the range of terabits per second may include 1000 multiplexed channels, utilizing a transmission frame of 1000 bits with one bit per channel, with each channel operating at a 1 gigabit per second data transmission rate, with a bit period (or bit time) of 10xe2x88x9212 seconds.
To enable such extremely high data rates in optical communications, however, various difficulties must be overcome which do not arise in lower speed communications. For example, due to external or environmental factors, the physical characteristics of optical fiber may change slightly, such as changes in the index of refraction and the physical length expanding or contracting depending upon environmental temperatures. At lower data rates, such changes may be inconsequential. At these higher terabit per second data rates, however, such physical changes may affect several bit periods, requiring new solutions to these new, heretofore non-existent problems. For example, due to the physical characteristics of optical fibers, thermal fluctuations may cause a relative drift, skew or other mismatch between the clock pulse and the desired OTDM data channel in a demultiplexer. At terabit data rates, such a drift or skew may cause errors in data transmission, such as erroneously interpreting a logical 1 as a logic zero, or may cause the wrong channel to be erroneously decoded, particularly should the drift span several bit periods. In contrast, such skewing in the picosecond range is irrelevant at current lower speed data rates, such as gigabit/second rates.
As a consequence, a need remains for an apparatus and method to maintain bit synchronization in optical transmission media. Such an apparatus and method should be able to track relative bit drifting or skewing, and maintain bit synchronization spanning several bit periods during a single transmission and without interruption of the transmission. In addition, such an apparatus and method should be capable of cost-effective implementation.
The apparatus and method embodiments of the present invention provide optical bit synchronization for optical communication and networking systems. The invention provides the capability to track relative bit drifting or skewing between clock pulses and selected data channels. The apparatus and method embodiments maintain such bit synchronization spanning several bit periods during a single transmission and without interruption of the transmission. The apparatus and method embodiments overcome a significant problem in optical communications, namely, the lack of available optical memory devices which are capable of data buffering and data storage at terabit per second data rates. In addition, the apparatus and method embodiments may be implemented cost-effectively.
The apparatus embodiment utilizes three main components, a programmable optical delay line, an optical synchronizer, and a processor (such as a microprocessor). The apparatus is preferably attached to an input of an optical gate, such as an optical demultiplexer, and is used to synchronize an input clock signal with an input data signal. Such synchronization is important both to properly select a data channel in an optical time division multiplexed system, and to correctly decode the data bit stream of the selected channel.
This problem of optical bit synchronization addressed by the present invention is new, and arises with previously unavailable and extremely high transmission rates, such as data rates in the terabits per second range. For such data rates, in accordance with the present invention, the synchronization of clock pulses has an extraordinarily small control resolution, such as within 0.5 picoseconds. Two control signals are utilized to accomplish this extreme precision, an offset signal and an interpolation signal. Continuously tracking a developing phase difference between the clock pulse and the selected data bit, the interpolation signal is utilized to delay or accelerate the clock pulse to keep it in synchrony with the data pulse. For example, the interpolation signal is utilized to continuously track comparatively small phase differences between the clock pulse and the data pulse, as these phase differences may develop. An offset signal is utilized to maintain such synchronization within the center of its physical range, particularly as the bit tracking may span one or more bit periods during a single communication session.
An apparatus embodiment of the present invention includes a programmable optical delay line couplable to an input clock; an optical synchronizer coupled to the programmable optical delay line; and a processor coupled to the programmable optical delay line and to the optical synchronizer. The processor includes program instructions to track bit synchronization between a clock pulse and a selected data bit during a communication session, generally utilizing the interpolation signal. When a bit tracking range of the optical synchronizer is approaching a predetermined limit, such that it will begin to exceed its dynamic range, the processor has further instructions to first, interrupt the communication session; second, return the bit tracking range to a zero offset (so that the optical synchronizer is in the center of its dynamic range); third, correspondingly adjust the timing of a programmable delay line; and fourth, resume the communication session.
Numerous other advantages and features of the present invention will become readily apparent from the following detailed description of the invention and the embodiments thereof, from the claims and from the accompanying drawings.